1. Field of the Invention
The present invention relates to a method for producing polycrystalline silicon. More specifically, it relates to a recycling-oriented method for producing polycrystalline silicon that, in producing silicon from silicon chloride by a method of reduction, allows a chlorine gas to touch an exhaust gas containing a reducing agent chloride gas generated as a by-product, an unreacted reducing agent and silicon particles in order to allow them to react with each other, and then separates and recovers a reducing agent chloride from the other impurities, while remarkably decreasing the amount of the impurities, and decreases the amount of waste generated as much as possible.
2. Description of Related Art
In these years, the lowering in the emission of carbon dioxide being thought of one of causative substances for global warming has increasingly been demanded in order to prevent the global warming. Therefore, it has been difficult to construct a thermoelectric power plant, and interest in photovoltaic power generation is growing as a technique to meet demand for new electricity.
In photovoltaic power generation, electricity is obtained from solar light using silicon-based solar batteries. As silicon used in solar batteries, substandard one of those for semiconductors has been mainly used, but if facilities for photovoltaic power generation are spread and demand of solar batteries is exponentially increased in future, there may be a fear that the supply of silicon becomes insufficient.
Therefore, in addition to the production of silicon for semiconductors, it is necessary to produce silicon for solar batteries. As one solution, a method of producing silicon from silicon tetrachloride using a zinc reduction technique is proposed, and there is proposed a method in which zinc chloride generated as a by-product is recovered after electrolysis as zinc and chlorine, and then zinc is used as a raw material for reduction of silicon tetrachloride whereas chlorine is used for production of silicon chloride (for example, see Patent document 1). However, in an exhaust gas discharged from a reactor for zinc reduction, not only a zinc chloride gas generated as a by-product but also an unreacted gas and silicon particles may remain, and if zinc chloride recovered by cooling and condensation of the zinc chloride gas is used for electrolysis as it is, the deterioration of electrolysis efficiency may be caused, and in an extreme case, electrolysis may be impossible. The use of zinc chloride generated as a by-product for electrolysis includes problems that a purification step like melt filtration or distillation is necessary, whose process is complicated even in any kind of purification step, and the amount of waste is increased because of residue.
Patent document 1: JP H11-92130 A (1999)